As a method replacing a conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor was proposed and has been practically employed. The method employs a radiation image storage sheet comprising a support and a stimulable phosphor layer (i.e., stimulable phosphor sheet), and comprises the steps of causing the stimulable phosphor to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as “stimulating rays”) to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The storage sheet thus treated is subjected to a step for erasing a radiation image remaining therein, and then stored for the next recording and reproducing procedure. Thus, the radiation image storage sheet can be repeatedly employed.
The stimulable phosphor emits stimulated emission when excited with stimulating rays after having been exposed to a radiation, and a practically used one gives a stimulated emission in the wavelength region of 300 to 500 nm when irradiated with stimulating rays in the wavelength region of 400 to 900 nm. A typical example of the stimulable phosphor conventionally used in a radiation image storage sheet is an alkali halide phosphor.
For example, Japanese Patent Publication Nos. 7-84588 and 7-84589 disclose CsX and RbX (X: halogen) phosphors. These publications, however, give any information about neither defects of the halogen atom and an amount of Eu2+ activator in the phosphor nor the valency of the activating element, particularly, valency of europium (Eu).
The radiation image storage sheet has a basic structure comprising a support and a stimulable phosphor layer provided thereon. If the phosphor layer is self-supporting, the support may be omitted. In general, a transparent protective film of polymer material is placed on the free surface (i.e., surface not facing the support) of the phosphor layer.
The phosphor layer usually comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor without binder. The phosphor layer containing no binder can be formed by deposition process or firing process. Further, the phosphor layer comprising agglomerated phosphor soaked with a polymer is also known. In any of these phosphor layers, the stimulable phosphor emits stimulated emission when excited with stimulating rays after having been exposed to a radiation such as X-ray. The radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the storage sheet in proportion to the applied radiation dose, and accordingly a radiation image of the object is produced in the storage sheet in the from of a radiation energy-stored image. Thus imagewise stored radiation energy can be released as stimulated emission by sequentially irradiating the storage sheet with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the signals.
The radiation image recording and reproducing method (i.e., radiation image forming method) is very advantageous as described above, and it is desired for the radiation image storage sheet employed in the method to have a high sensitivity and give an image of high quality (high sharpness, high graininess).